Hyperthermal molecular DABCO [N^H^N] is scattered from hydrogen-covered Pt(lll). Some of the scattered molecules are protonated at the surface and leave with a kinetic energy which is strongly dependent on the incident energy. This means that proton abstraction is occurring immediately on collision and it serves as a clear demonstration of an Eley-Rideal mechanism. No isotope effect was observed, excluding a tunneling mechanism. The proton transfer shows a threshold energy equal to the difference between the surface work function and the molecular ionization potential. The reaction therefore proceeds via a molecule-surface electron transfer and a subsequent H-atom abstraction.
Surface ionization of molecules with hyperthermal kinetic energy (1-20 eV) was found to be very efficient, demonstrating several unique features. We have used the technique of aerodynamic acceleration in supersonic seeded beams in order to obtain molecular kinetic energies in the range 1-20 eV. In this energy range, scattering events are impulsive, being free from molecular adsorption on the surface, and surface ionization occurs under nonthermal equilibrium conditions. Thus, in hyperthermal surface ionization (HSI), the kinetic energy is directly converted either into the energy difference between the surface work function and the molecular electron affinity (for negative ions) or into the molecular ionization potential minus the surface work function (for positive ions). Four types of HSI processes were observed, (a) Surface-molecule electron transfer was demonstrated in the I2/diamond system where negative molecular iodine (I2-) ions were produced, (b) Molecule-surface electron transfer was found for the anthracene molecule where positive molecular anthracene ions were generated, (c) Abstractive ionization was detected for lV,7V-dimethylaniline (DMA) scattered from diamond. A protonated molecular ion was observed, (d) Hyperthermal surface induced dissociative ionization (HSIDI) was observed in 1-iodopropane, benzyl bromide, and many other molecules. In these processes we have observed a large current of negative halogen ions and positive molecular residue ions (ion-pair formation). Hyperthermal surface ionization is characterized by several experimental features such as the following: (a) A very large dependence of the ionization yield on the incident molecular kinetic energy. HSI has an energetic threshold which depends on the molecule, the surface, and the ion. (b) The negative ions were generated on diamond and not on the "technical" metal that served as a support for the diamond crystal. The positive ion yield was larger on this metal support, (c) The effect of the surface temperature on the ionization yield was small, (d) The HSI yield decreased with the beam-surface incident angle, (e) The angular distribution of the scattered ions was shifted toward grazing angles (supraspecular scattering), (f) The ions energy distribution was broad, structured, and non-Boltzmann, (g) HSIDI resulted in the generation of ions whose yield correlated with the generation of neutral atoms. The mechanism of hyperthermal surface ionization (HSI) is described in terms of electron-transfer processes. It occurs due to a curve crossing between the neutral scattering interaction potential surface and the ionic interaction potential surface, and a nonunity reneutralization second curve crossing of the scattered molecules or fragments. The HSIDI mass spectra demonstrated a highly nonstatistical mass spectral fragmentation pattern. This fragmentation pattern was largely different from that of the electron impact ionization mass spectra. It was affected by the fragment electron affinity, by the ionization potential of the molecule o...
The mechanism of ambient pressure encapsulation of He and Ne in the R and β crystalline cages of type-A zeolites is demonstrated. Reversible and highly selective gas admission and entrapment are readily achieved at characteristic temperatures occurring between 77 and 570 K. The permeability of the zeolitic windows is governed by an interplay between the critical diameter of the encapsulate and the effective apertures dimension, which is shown to be strongly dependent on temperature. The blocking state of the zeolitic apertures is determined by a simultaneous thermal activation of both cation mobility and structural dilation/constriction of crystalline windows. Encapsulation in NaA (4A) principally occurs in the β cages of the Sodalite units, whereas the K-exchange form (3A) offers both R and β encapsulations. The effective free aperture dimension of the Ca exchange form (5A) is found to be too large to allow a practical gas enfoldment in either class of cavities, even at 77 K, where only poor encapsulation is observed. The counterion location vs size dependence, known only from crystallographic data, is sensed here for the first time by an encapsulation process, via the manifestation of different aperture occupancy states. While the blocking extent of the wider O 8 windows of the R cages is consistent with the size of exchangeable cations, a reverse correlation is evident for the narrower O 6 windows of the β cages.
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